The present invention relates to centrifugal clutches such as are used in light vehicles, chain saws, pumps, wind mills, compressors, engine, electric motor, wind driven products and the like for variably coupling rotational power sources to driven loads.
Centrifugal clutches are disclosed, for example in U.S. Pat. Nos. 2,942,711 and 3,971,463 to the above-named inventor, which patents are incorporated herein by this reference. Typically, a plurality of radially movable shoes frictionally contact an inside surface of a drum that rotates concentrically with a hub to which the shoes are connected. Normally, the hub is fixed on a driving shaft or face plate that is coupled to an engine or power source, the drum being coupled to a driven load by means such as a belt, chain or direct coupled drive. Such clutches transmit little or no torque at low speeds, and progressively increased torque at higher speeds in proportion to centrifugal forces to which the shoes are subjected. In the above-identified patents, spring biasing elements are employed for maintaining the clutches in a disengaged condition at speeds below a predetermined threshold for permitting the engine or other power source to idle at no load.
Centrifugal clutches as implemented in the prior art are subject to a number of problems and disadvantages. For example:
1. They are ineffective in that they fail to provide a desired torque-speed relationship to match the characteristics of the power source; PA1 2. They are short-lived in that there is excessive slippage and consequent wear of the shoes, particularly when there is a large vibrational component or high inertia load in the torque; PA1 3. They are unreliable in that springs used therein are subject to breakage, particularly in that they have fatigue inducing stress risers in hooks at opposite ends; PA1 4. They are unsafe in that the shoes can fly apart in case there is structural failure of the drum; and PA1 5. They are undesirably expensive to produce.
The clutch as disclosed in the '711 patent can be assembled in a variety of ways to selectively provide correspondingly different torque-speed relations such as straight-centrifugal with selectable speeds of engagement; relatively quick-acting; or relatively smooth-acting. However, none of the torque-speed relations is entirely satisfactory, and there are numerous possibilities for incorrect assembly. The clutch as described in the '463 patent provides driving member projections that are successively engageable with the shoes at increasing speed increments for progressively increased torque transmission. Again, none of the torque-speed relations is entirely satisfactory, particularly when it is desired to obtain initial engagement at a high percentage of the full-load speed of the clutch, and the clutch is excessively complex and expensive to produce.
It is also known to have direct metal-to-metal contact between the drum and metal shoe members of centrifugal clutches. However, the applications for such clutches are severely limited by a lower coefficient of friction, excessive wear, and galling.
Thus there is a need for a centrifugal clutch that automatically provides a desired torque-speed profile that can smoothly transition from no-load to full load in a narrow speed range, that is fail-safe against separation of the shoes in case of failure of the drum, and that is reliable, long-lasting, and inexpensive to produce.